In Experiments 1 and 2, rats were exposed to two compound flavors, AXand BX, containing one flavor in common (X). Following this exposure phase, an aversion was conditioned to A in the experimental group by pairing its consumption with an injection of lithium, while a control group drank A without being poisoned. The effect of this treatment was to establish B as a conditioned inhibitor. In Experiment 1, experimental animals were slower than controls to condition an aversion to B when its consumption was paired with lithium (a retardation test of conditioned inhibition). In Experiment 2, B alleviated the suppression of intake of another flavor previously paired with lithium (a summation test). Experiments 3 and 4 established that these effects depended upon prolonged prior exposure to AX and BX.In their theory of stimulus representation, McLaren, Kaye, and Mackintosh (1989) suggested that exposure to two compound stimuli (AX and BX), containing a common element X, would permit the establishment of two types of associations between the elements of each compound. The first would be excitatory associations between the common element X and each unique element, A and B. The existence of such associations is evident from studies such as that of Rescorla and Cunningham (1978), employing compound flavors. Their effect will presumably be to enhance generalization between the two compounds: if AX is paired with an unconditioned stimulus (US), the ability of X to retrieve a representation of B could result in excitatory conditioning to B as well as to A on this trial. Acquired equivalence effects of this sort have been demonstrated by Hall (1989, 1991).It is well established, however, that prolonged exposure to AX and BX will often facilitate subsequent discrimination, rather than increase generalization between them (Hall, 1991). McLaren et al. (1989) attribute this perceptual learning etTect, in part, to differences in the associability of the common (X) and unique (A and B) elements, which occur as a consequence of greater latent inhibition of the former than of the latter. Several studies have provided good evidence for such a suggestion (e.g., Mackintosh, Kaye, & Bennett, 1991; Rodrigo, Chamizo, McLaren, & Mackintosh, 1994) see Experiments 3 and 4) concluded that such differential latent inhibition of common and unique elements could not be the only explanation of perceptual learning. They suggested that one additional mechanism was provided by a second set of associations that are formed between the elements of AX and BX-~specifically, inhibitory associations between A and 8. The establishment of excitatory associations between common and unique elements may cause X to retrieve a representation of B on AX trials (and of A on BX trials), but the presence of A signals the absence of the otherwise expected B (just as the presence of B signals the absence of the otherwise expected A), and according to standard associative theory, this should lead to the eventual establishment of inhibitory associations between A and B...

Forward and backward blocking of taste preference learning was compared in rats. In the forward condition, thirsty rats were exposed to a flavor (A) in sucrose solution (+) or in water (-), after which they were exposed to A in compound with another flavor (B) in sucrose solution (i.e., AB+). In the backward condition, these phases were reversed. Consumption of B alone was assessed when rats were food deprived. In the forward condition, rats given A+ consumed less B than rats given A-, providing evidence of forward blocking, whereas in the backward condition, rats given A+ drank more of B than those given A-. Subsequent experiments found that alternating but not blocked preexposure to A and B, when given prior to training, produced blocking of B whether A+ was given before or after AB+, suggesting that prior failures to observe backward blocking reflect failures of discrimination.

When they are trained in a Morris water maze to find a hidden platform, whose location is defined by a number of equally spaced visual landmarks round the circumference of the pool, rats are equally able to find the platform when tested with any two of the landmarks (Prados, & Trobalon, 1998; Rodrigo, Chamizo, McLaren, & Mackintosh, 1997). This suggests that none of the landmarks was completely overshadowed by any of the others. In Experiment 1 one pair of groups was trained with four equally salient visual landmarks spaced at equal intervals around the edge of the pool, while a second pair was trained with two landmarks only, either relatively close to or far from the hidden platform. After extensive training, both male and female rats showed a reciprocal overshadowing effect: on a test with two landmarks only (either close to or far from the platform), rats trained with four landmarks spent less time in the platform quadrant than those trained with only two. Experiment 2 showed that animals trained with two landmarks and then tested with four also performed worse on test than those trained and tested with two landmarks only. This suggests that generalization decrement, rather than associative competition, provides a sufficient explanation for the overshadowing observed in Experiment 1. Experiment 3 provided a within-experiment replication of the results of Experiments 1 and 2. Finally, Experiment 4 showed that rats trained with a configuration of two landmarks learn their identity.

In two experiments rats were preexposed to neutral stimuli. Both experiments used a between-subjects design in which a paired group was preexposed to intermixed presentations of A --> Band AX, and an unpaired control group was preexposed to intermixed presentations of A, B, and AX. After the conditioning of B, in Experiment 1, conditioned responding to X was acquired more slowly in the paired than in the unpaired group. Furthermore, in Experiment 2, X reduced conditioned responding to a separately trained excitor in a summation test but only in the paired group. Together, these results provide evidence of an inhibitory form of sensory preconditioning.

In the present study the authors sought to establish whether the range of effects of neonatal handling stimulation (H), that is, brief daily periods of infant isolation, could be extended to the domain of social motivation. With this aim, the authors studied the innate motivation to engage in rough-and-tumble play (R&T) in adolescent rats (Rattus norvegicus) by means of a reversal design, in which half of the rats were first housed in isolation (Days 1-3), and then in company (Days 4-6), while the other half followed the reverse sequence of housing conditions. Results showed in a clear-cut manner that H fuelled playfulness, as measured by pin and dorsal contact episodes, with (relative) independence of trait-based differences in fearful behavior between handled and nonhandled rats. Given that the different levels of the rat's social brain are apparently sensitive to tactile stimulation in infancy, the authors propose that the vibrant R&T reported here could reflect an enduring alteration of genetically based, motivational systems underlying playfulness and, perhaps, positive social emotions like joy.